Quoting Jaroslav Kysela <[EMAIL PROTECTED]>:
> I don't think so. It seems that my brain still remembers assembler ;-)
...
> sample = *sum;
> s16 s;
> - if (unlikely(sample & 0xffff0000))
> + if (unlikely(sample & 0x7fff0000))
> s = sample > 0 ? 0x7fff : -0x8000;
> else
> s = sample;
I think I remember some of the x86 assembly myself and this correction
does not fix the problem. This code will still "saturate" all negative
samples to -8000. You cannot detect an overflow into the upper half of
the register with a simple bitwise and. The actual test should be as
follows :
- extend the sign of the lower half
- check if the upper half is the same as the effect of expansion
if it is - there is no overflow
if it differs - there was overflow and you need to saturate.
examples :
value 0x 0000 0335
ext 0x 0000 0335
-> no overflow
value 0x 0002 43b1
ext 0x 0000 43b1
-> overflow
value 0x ffff f25b
ext 0x ffff f25b
-> no overflow
value 0x ff1c 35c9
ext 0x 0000 35c9
-> overflow
to put it in asm:
mov ebx,eax
cwde
cmp eax,ebx
The problem is cwde operates only on ax/eax.
This may sound complicated but in fact it amounts to a very simple
question : does the sample fit in a 16 bit int, or does it not, so
I guess in C it could look something like :
s16 s=sample;
if (unlikely(sample != (s32)s))
The cast is just there for clarity I believe it would be done
implicitly anyway. But don't take my word for it - I did not
test this.
--------------
Fycio (J.Sobierski)
[EMAIL PROTECTED]
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